Sanding painted MDF?

[Derek Larson]

Good morning all,
I am just about to complete my first wood working project. I made a book shelf out of MDF, I know alot of you are cringing at the use of MDF for a book shelf so let me quickly explain why.
I am using MDF because this is my first real wood working project and I wanted to practice some basic skills like creating dados for the the shelves and a simple face frame (which should add some support to the MDF shelves), but I didnt want to practice on more expensive plywood or lumber (and my wife wanted a white bookcase). So I chose to use MDF.
Now I unfortunately I do not have a sprayer so I had to basically use a roller to paint the parts of the bookcase. I had been sanding between coats to get a nice smooth finish and last night I put the last coat on. So I want to sand the last coat smooth but am concerned that when I do this it will lose its sheen. So what grit paper should I use and should I use a different technique such as using a wet sanding method?

Thank you

[Prashun Patel]

IMHO, paint isn't meant to be 'rubbed out' like clear finishes. You might improve adhesion by sanding between coats, but the final coat should be left alone. If you're getting roller nap marks, you might switch to a low-nap roller or even a foam roller. You can also try thinning the paint a little with a compatible solvent. This will allow it to flow and level itself a little better before setting up.

A couple things to note for your next project:

1) Latex paint (if that's what yr using) is not the best choice for a bookcase shelf. It cures 'soft' and heavy objects can stick to it. Oil or urethane paint is a better choice.

2) MDF is a perfectly fine choice for ww projects, but it has structural limitations. If those are understood, it's fine for what yr doing.

3) You can spray a compatible aerosol varnish on top for durability and sheen.

[Bill White]

Penetrol and Flotrol are additives. One for latex, one for oil. Reall helps the paints lay down smoothly. Check with the paint store, and use a good brush if you're not spraying.
Bill

[Derek Larson]

Great advice guys I will keep this in mind for the next project, and for this project I think I will take Shawns advice. I will lightly sand what I have done the spray it with an aerosol varnish. And for future projects I will try the additives that Bill mentioned. Thanx for helping a complete amateur.

[Chris Padilla]

Polycrylic (polyurethane) will also work as a final coat over the latex to prevent "blocking" (that is the stickyness of latex observed even months after drying and sometimes even years depending upon what is placed on it).

Higher quality paint like Sherwin-Williams will also serve you better, flow better, and simply look and work better. I know ~$50/gal paint might make you cringe but when you see the results, you'll see it is worth it.

[Frank Drew]

Enamel (varnish) based paints were actually rubbed out, or polished, in the very best painted work, but that's a practice long past. The traditional high end high gloss paint job was an elaborate multi-step, multi-coat process.

[Prashun Patel]

Great advice guys I will keep this in mind for the next project, and for this project I think I will take Shawns advice. I will lightly sand what I have done the spray it with an aerosol varnish. And for future projects I will try the additives that Bill mentioned. Thanx for helping a complete amateur.

Make sure you check the compatibility. Also, beware that some varnishes can add an amber cast to your project. I'm no expert, but have done this on some closet shelves (polycrylic over latex) and it worked fine. TEST FIRST.

[Shawn Walker]

A high quality foam roller on your last coat will give a nicer finish.
I'm talking about the mini foam rollers... They are 3 or 4 in. long x around 1 1/2 thick. They sell them AT HD.
I agree about the Oilbase for a good finish and durability.

[Chris Padilla]

Do not use Polycrylic with a foam roller. I had air bubbles galore show up. Perhaps I rolled too fast but I dunno. Thankfully, it was on a piece I didn't care about but lesson learned...I think.... :D